Drilling fluids used in the drilling of subterranean oil and gas wells along with other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. The drilling fluid should carry cuttings from beneath the bit, transport them through the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that as defined herein, the term “drilling fluid” also encompasses “drill-in fluids” and “completion fluids”.
Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water. Nonetheless, the water is the continuous phase. Oil-based muds are the opposite or inverse. Solid particles are suspended in oil, and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds which are water-in-oil macroemulsions are also called invert emulsions. The oil in oil-based (invert emulsion) mud can consist of any oil that may include diesel, mineral oil, esters, or alpha olefins. Brine-based drilling fluids, of course are a water-based mud in which the aqueous component is brine.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. Because drilling fluids are called upon to perform a number of tasks simultaneously, this desirable balance is not always easy to achieve.
Filter cakes are the residue deposited on a permeable medium such as a formation surface when a slurry or suspension, such as a drilling fluid, is forced against the medium under pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in a wellbore can cause stuck pipe and other drilling problems. Reduced hydrocarbon production can result from reservoir or skin damage when a poor filter cake allows deep filtrate invasion. In some cases, a certain degree of cake buildup is desirable to isolate formations from drilling fluids. In open hole completions in high-angle or horizontal holes, the formation of an external filter cake is preferable to a cake that forms partly inside the formation (internal). The latter has a higher potential for formation damage. It will be appreciated that in the present context the term “filter cake” includes any emulsion or invert emulsion part of the filter cake, and that the filter cake is defined as a combination of any added solids, if any, and drilled solids. It will also be understood that the drilling fluid, e.g. invert emulsion fluid, is concentrated at the bore hole face and partially inside the formation. Further, an open hole completion is understood to be a well completion that has no liner or casing set across the reservoir formation, thus allowing the produced fluids to flow directly into the wellbore. A liner or casing may be present in other intervals, for instance between the producing interval and the surface.
Many operators are interested in improving formation clean up after drilling into reservoirs with invert emulsion drilling fluids. More efficient filter cake and formation clean up is desired for a number of open hole completions, including stand-alone and expandable sand screens as well as for gravel pack applications for both production and water injection wells. Skin damage removal from internal and external filter cake deposition during oil well reservoir drilling with invert emulsion drill-in and drilling fluids is desirable to maximize hydrocarbon recovery, particularly in open hole completions.
Further, it is often desirable in the destruction and removal of invert emulsion filter cake to not do so quickly, but rather to delay the destruction and removal of the filter cake. Without control of the destruction rate, massive brine losses may occur quickly and before the work string can be safely pulled out of the open wellbore.
It would be desirable if compositions and methods could be devised to aid and improve the ability to clean up filter cake, and to remove it more completely, without causing additional formation damage. It is also desirable to control the rate of destruction and removal of the filter cake.